1. Field of the Invention
This invention relates to the recovery of precious metals from scrap materials and more particularly to the recovery of gold, such as in the form of visible gold plate, from printed circuit board scrap material and the like and wherein the gold is recovered in flake form without being dissolved.
2. Description of the Prior Art
Prior art methods for recovery of gold from scrap generally comprise acid dissolution or cyanide dissolution. Both of these methods include as an initial step dissolution of the gold to be recovered. The "scrap" gold is dissolved in an appropriate solution to yield a soluble salt. The choice of salts produced depends entirely on the solubility of the metal anion with the cation. Hence, in the case of acid dissolution the gold is held in solution as soluble gold chloride (AuCl.sub.2) and in the case of cyanide is held in solution as soluble gold cyanide (AuCN).
Subsequent to dissolution of the gold in this manner either metallic copper or zinc dust is added to the soluble gold salt-containing solution to displace the gold from the salt solution thereby producing free metallic gold and either soluble cupric chloride or zinc cyanide, displacement being dictated by the more electronegative element.
While the chemical steps and reactions attendant acid or cyanide dissolution are essentially very simple, the practice or such methods results in the creation of several significant attendant problems. Foremost among these prior art processes is an alloy of all metallic elements present in the scrap that are soluble in either aqua regia, or chlorine in the presence of oxygen, and cyanide solution. Thus, when using an aqua regia bath, iron, nickel, copper, zinc, cadmium and other elements and alloys such as, bronze will be present wherein an impure gold base alloy will be recovered. With respect to a cyanide dissolution method the recovered gold ingot will generally comprise an alloy of gold with copper, zinc, silver, cadmium and bronze as carry-over elements or alloys.
It will thus be apparent that in order to obtain pure gold by these prior art methods the gold alloy ingot requires further refining. This normally dictates that relatively expensive electrolytic methods must be utilized to derive gold from the gold alloy ingot.
It is recognized in the prior art, see U.S. Pat. No. 4,261,738, that gold may be recovered from scrap materials and wherein the process disclosed therein recovers gold from gold filled scrap as karat gold useable directly in the formation of gold filled blanks and metallic stock which eliminates the step of realloying. Base metal intimately bonded to the gold is leached off with an ammoniacal solution of an ammonium salt using a leaching liquid containing reactive oxygen. The base metal or metals are recovered as metal compounds by thermally decomposing the resulting pregnant leach liquor and the ammonia and ammonium salt are reconstituted for recycling. While the patent teaches that silver and any nickel adhering to karat gold recovered by the ammoniacal leach step may readily be removed from the karat gold by treatment with nitric acid with subsequent filtration for recovery of karat gold, the process disclosed therein is inordinately complex and thus considered to not be cost effective or practicable for situations where relatively small amounts of gold potentially may be recovered from an electronic component manufacturing operation.
The principal object of the present invention is to provide an improved simple, economical method for the recovery of visible gold plate, in pure or alloyed form, such as adhered to a support lamina, e.g., printed circuit board scrap material and the like, by an underplating such as comprising copper, nickel, tin and alloys thereof.
It is another object of the present invention to provide an improved method for the recovery of gold plate wherein the gold plate is never dissolved.
Still another object of the present invention resides in an improved method for the recovery of visible gold plate wherein the gold plate is physically divided into "flakes" to facilitate recovery of the gold by filtration for recovery of gold of substantially the analysis of the gold plate of the scrap material being treated and wherein such recovered concentrate may then be furnace fired to produce a high purity ingot or button.
A further object of the invention resides in an improved method and composition for the recovery of visible gold plate wherein the gold is chemically and physically removed from the scrap material by an aqueous etching solution having froth creating compounds incorporated therein.